The present invention relates to a universal electric motor and an automated method of producing such a motor.
A problematical feature of a known universal motor of this type (DE-OS 37 31 079), with clockwise and counterclockwise rotation and intended for electric hand tools, is the fact that, for the purpose of operating the reversing switch, two contact springs exerting pressure in a radially inward direction are arranged on the circumference of a carrier ring; each of said springs is electrically connected to a brush-holder and at their neighboring but separate ends they each have a mains supply contact point and a field contact point. The pressure necessary for a reliable contact is therefore produced solely by the radially acting force of the springs, which are arranged within the axial space constituted by the carrier ring, a space that also contains the carbon brushes, and the possibilities of fouling and contact difficulties cannot therefore be excluded. The carrier ring of this arrangement is rotatably mounted on a central axial projection of the adjacent end shield and is maintained in its axial position by means of a retaining ring.
In connection with the switching of electric hand tools from clockwise to counterclockwise rotation it is further known (EP-PS 0208137) to design the device for displacing the brushes in contact with the commutator as a reversing switch in the form of two carrier bodies made of insulating material that can be coupled together, the two carrier bodies comprising interlocking collars that constitute a hollow space in which the contact elements for the reversal are arranged. This known hand tool also lends itself to substantially automated assembly, because the two carrier bodies are designed in the form of a contact holder for the contacts for the external connection of the hand tool or as a contact plate with the brush-holders and the contacting with the stator connections is obtained by means of axial contact lugs. Switching processes for changing the direction of rotation are made possible by the insertion of radially spring-loaded contact bands in holder pockets on the contact plate, so that contact, just as in the case of the previously considered electric hand tool of DE-OS 37 31 079, is once again produced in a radial direction.
In another known electric driving motor (DE-OS 31 49 106) the stator, which is constituted by a permanent magnet, is surrounded by an annular strip of metal, thereby creating a magnetic short-circuit, the said strip being on both sides axially adjacent to bearing holders and connected to them. These bearing holders are designed in the form of stirrups and each of them accommodates a bearing of the rotor shaft. One of these bearings is in the form of a roller bearing that is held directly in a cup-shaped hollow of the bearing holder and projects outwards from this recess by a predetermined amount. Consequently, it is possible to use this bearing element and, more specifically the outer ring of the roller bearing, which is necessarily perfectly centered with respect to the rotor shaft, for anchoring and centering other components attached to this motor. Nothing is said about the possibility of electrical switching; however, greater costs can hardly be involved here, since the stator consists of a permanent magnet.
It is further known (U.S. Pat. No. 4,613,781) to arrange an end-cap assembly unit on one side of an electric motor, the said unit comprising a plastic component in the interior of which there is embedded a large number of strip-shaped metallic conductors that establish the electric connections between the motor components and the brush-holders. To this end each electric line that is to make contact with a brush-holder is wound around a base area of the brush-holder, for which purpose the base of each brush-holder is provided with lateral flanges.
Further, the individual brush-holders are so designed that a side wall is bent away in the manner of a loop and is so extended as to make it reach under the carbon brush in the brush-holder in the manner of a hook, thereby enabling it for the time being to hold this brush--for assembly purposes--at a distance from the commutator ring of the rotor. Subsequently, i.e. after assembly, the hook is pressed further outwards, so that the carbon brush is released and it becomes possible for electric contact to be established between the brush and the commutator of the electric motor.
As regards components for an electric motor, it is further known (DE-GM G 85 20 258.4) to provide a plate made of insulating material and carrying a circuit arrangement, the said plate having a central opening to accommodate the commutator of an electric motor, and to provide on this plate, arranged diametrically opposite each other, single-piece frame parts to hold carbon brushes that can be connected to the circuit arrangement, the said frame parts being designed in such a manner that spring-pretensioned prolongations of the spring lead from spring-holders arranged by the side of the frame parts into the frame parts themselves. The springs are wound onto the spring-holders in the manner of a screw. At least a part of the electric circuit arrangement is embedded into the plastic circuit board.
This embedding of at least the essential components of the wiring needed for the operation of an electric motor is also known from DE-OS 36 04 675, where an insulating component surrounds the essential components of the conductor strip and also supports other components for the operation of the motor, brush-holders for example, which are likewise integrated into the component. The component is designed as a plate and has a central opening to provide a passage for the commutator of the motor.
In another electric motor of known construction mode (European Patent Application 0235443) a large number of electric components are arranged on an annular base board, the components making contact with the embedded electric conductors by virtue of the fact that the board partly bares these conductors for contacting purposes. The brush-holders, likewise carried on this board, are provided with extensions on the side of the edge that are clipped onto free parts of the conductors, so that it is possible to establish a secure physical connection between the conductors within the board and the brush-holder housings. The sole assembly costs incurred in arranging and supporting such a large number of individual components on the board, including separate, wound brush tensioning springs and the pins that carry them, are already considerable. Parts of the electric conductors embedded in the board project in this case beyond it in the axial direction, thereby making it possible to create contacts at other places, with the stator for example.
It is also known to design a beaker-shaped bearing bridge in a motor for an electric tool in such a manner as to enable it to accommodate the stator that is slid into it, the stator, in its turn, accommodating in its interior the freely rotatable rotor. On the side of the beaker-shaped bearing bridge the rotor shaft is seated in a roller bearing that is constituted by the bearing bridge. The bearing bridge further disposes of means for switching on and off, field plugs, and a radio interference suppression condenser. Arranged on the bearing bridge there is a pattern of contacts, which is designed as a punched grid for the optional connection of chokes, reversing switches, diodes and resistances. This punched grid is situated on the rotor-facing side of a switch array. A switching ring is also provided, and various contacts can be established as this ring is rotated, including contacts for direction reversal or speed reduction (European Patent Application 0224054).
In this connection it is also known (European Patent Application 0224054) to arrange the planar punched grid on one side of a brush-holder plate, with the carbon brushes situated on the other side of the brush-holder plate. Connection contacts for the field windings are obtained by means of connection lugs set at right angles to the principal plane of the punched grid. The punched grid in this particular case is designed as a continuous metal strip conductor and welded, glued or otherwise attached to the rotor-sided surface of the brush-holder plate, which is made of plastic material.
The problematical aspect of the known electric motors and/or components for them is that even though simplifying solutions are being obtained as regards certain individual features, the structures are still substantially too complex when one considers the cost incurred in respect of the individual parts, fitting them together and assembling the motor, especially when one bears in mind that such an electric motor should also have means to permit switching on and off and, above all, reversing the direction of rotation. But direction reversal, in particular, is normally associated with considerable circuit costs, so that problems that seem to bar the way to substantially automated assembly and low-cost production of such motors arise not only here, but also in connection with providing bearings for the brushes and the creation of good contacts between them and other circuit parts and, quite generally, in the entire basic structure and design of electric motors.